1. Field of the Invention
The invention relates to high frequency welding of synthetic materials such as vinyl, polyester, nylon, PVC, plastic films, and the like. More particularly, the invention relates to connectors for coupling electrodes used in high frequency welding.
2. State of the Art
It has been well known in the art for many years to use high frequency electrodes to weld plastic sheet material. The underlying theory in this type of welding is known as dielectric or radio frequency heating. Almost any material will heat if an electric current of sufficient intensity is passed through it. The amount of heating is related to the conductivity of the material, the force (voltage) of the current passing through the material, and the frequency of the current. In the case of very poor conductors, such as most plastics, the amount of force required to heat these materials with a 60 Hz current is on the order of millions of volts. However, it is known that as the frequency of the current increases, the conductivity of the material being subjected to the current increases also. This is because the rapid change in the polarity of the current causes the molecules in the material to rapidly change their orientation thereby generating heat inside the material. Since materials which are poor electrical conductors are generally poor thermal conductors as well, radio frequency heating is the preferred method for sealing and welding these materials.
Typical applications of RF welding involve the welding of two plastic sheets. Prior art FIG. 1 shows an example of a simple sealing bar (rule) electrode 10 sealing two sheets of PVC film 12, 14. The two sheets of film 12, 14 are placed on top of each other and on top of a bedplate electrode 16. An optional heat insulating buffer material 18 is placed between the bedplate electrode 16 and the lower sheet 14. The buffer material 18 increases the effectiveness of the sealing apparatus by reducing heat sink losses through the bedplate electrode 16. The electrodes 10 and 16 are typically made of brass because brass is a good conductor, is durable, and is easily machined. In some applications, electrodes are made of beryllium copper (for extended runs ) or aluminum and steel ( for short runs ) . The buffer material 18 is typically a sheet of 0.015 XXP or XXXP grade, paper base, natural color, phenolic material because it is a sufficient thermal insulator, does not interfere with the electrical field passing through it, and is not heated directly by the field. After the sheets 12, 14 and the buffer material 18 are arranged on the bedplate electrode 16, the electrode 10 is pressed against the upper sheet 12 under the action of an air press (not shown) and RF current is caused to flow between the electrodes 10 and 16. The portions of the sheets 12, 14 which are located between the electrodes 10, 16 are caused to heat from the inside out whereupon they melt into each other and form a seal 20 which has substantially the same size and shape as the surface of the electrode 10.
In many cases, it is the outer edges of two sheets of plastic which are to be welded. This is the case, for example, in the manufacture of vinyl covered notebooks and the like. Moreover, it is also often desirable to provide a decorative bead effect at the outer edge of the welded sheets. Prior art FIG. 2 shows a "tearseal" electrode 11 which has an outer sharp edge 11a, an inner sealing surface 11b, and an intermediate bead well 11c. The outer sharp edge 11a typically extends beyond the inner sealing surface 11b by an amount approximately equal to one half the combined thickness of the sheets and the inner sealing surface 11b typically extends beyond the intermediate bead well 11c by a similar amount. The electrode 11 may be made from a laminate of three pieces of brass or may be machined from a single piece of brass. The operation of the sealing apparatus shown in FIG. 2 is substantially the same as the one in FIG. 1. However, as the sheets are welded, the outer sharp edge 11a substantially cuts the outer edges of the sheets and after the sheets are welded, the excess material is neatly removed by tearing it away.
In many applications, it is necessary or desirable to weld two sheets of plastic along two or more edges simultaneously, usually along four edges. FIG. 3 shows one known method of configuring electrodes to make a rectangular edge weld. Four brass rule electrodes 13, 15, 17, 19 are mounted on a backing plate 21. The backing plate 21 is typically a cast aluminum tool and jig plate. Cast aluminum is preferred because it is light weight, has a flat surface, and it cuts and taps easily. The electrodes are mounted to the backing plate 21 using aluminum mounting blocks 23, 25, 27, 29, 31, 33, 35, 37. Each block has a number of non-interfering holes disposed at right angles to each other. Each electrode is fastened to one or more blocks with screws or bolts and the blocks are fastened to the backing plate using screws which engage holes tapped in the backing plate. It will be appreciated from FIG. 3 that in using brass rule electrodes, the corners of the rectangular edge weld will be sharp right angles since the rule electrodes meet at right angles. Moreover, unless the corner connections of the rule electrodes are beveled, the resulting weld will be uneven. It is usually desirable, however, to provide a weld with rounded corners so that the corners of the edge weld will be smooth, attractive, and more resistant to separating.
Prior art FIG. 3a shows one of the presently preferred methods of providing rounded corner edge welds. Four rule electrodes 39, 41, 43, 45 are each provided with a curved right angle bend 39a, 41a, 43a, 45a by working the brass electrodes around a curved bending tool. The electrodes are then mounted to the backing plate as described above. While this method provides rounded corners, it leaves seams s1, s2, s3, s4 between each electrode along their straight edges. Another method of providing rounded corners in a rectangular edge weld electrode is to take two single rule electrodes, bend each two times and swage/the ends. Both methods require the use of a machine shop and involve considerable time and effort.